Skylight and box gutter leaks: causes and fixes

Published 24 September 2026 · Updated 13 October 2026

Skylight leaking, or is it condensation?

Check when it drips. Water that appears only during or after rain, especially wind-driven rain from one direction, points to a leak. Water on the glass or running down the shaft on cool mornings with no rain points to condensation: warm, moist air from a bathroom, kitchen or bedroom meeting cold glass.

VELUX, a skylight manufacturer, says that if condensation forms on the glazing “you should take it as a warning sign that you should ventilate more often”, and that “Regular ventilation is the most efficient way of preventing condensation water from forming on your window glazing.” An exhaust fan in a bathroom with a skylight helps. A roofer can’t fix condensation from the roof.

Why skylights leak

• Leaves and debris above the skylight. On a tile roof under gums, leaves pile up against the upper flashing and dam the water. VELUX’s care guide says “Leaves and other debris should be re moved from the flashing around the window once per year” so rainwater can flow freely.
• Lifted or corroded flashing. Near the surf, a steel flashing or its screws can rust through before the roof does.
• Cracked or slipped tiles around the skylight, from foot traffic or a falling branch, letting water in beside the flashing.
• Old sealant repairs. Silicone over a gap is a stopgap: it cracks with sun and movement, and can trap water rather than shed it.
• A re-roof or other roof work that disturbed the skylight’s flashing kit, or used flashings from a different system.
• Storm damage to the glazing or dome, such as hail or a branch.

Rather than sealing over a leaking skylight, a roofer usually lifts the surrounding tiles or sheets, replaces or refits the flashing kit, and re-lays the roof around it.

Why box gutters leak

A box gutter sits inside the roof line rather than hanging off the edge: between two roof sections, behind a parapet, or along a terrace or a block of units. If an eaves gutter overflows, the water falls outside. If a box gutter overflows, it goes into the ceiling and down the walls. That’s why a box gutter leak often shows up as a stain along a wall, or a drip in the middle of a room under a valley between two roofs.

The usual causes on the Beaches are leaf litter blocking the outlet or sump, rusted joins and laps (salt sits in a box gutter’s sheltered corners), a gutter too small for the roof area draining into it, and no overflow to carry water out of the building when the outlet blocks. The NSW SES warns that “Blockages can trap water and cause it to overflow under the eaves into your roof cavity”, and a box gutter has no outside edge to overflow from.

What the building code says

The National Construction Code’s simple rules for gutters and downpipes (Housing Provisions Part 7.4) don’t cover box gutters: NCC 2022 Volume Two clause H2D6(3) says that part “excludes box gutters”. Box gutters are designed to the plumbing standard AS/NZS 3500.3 instead, including how water escapes when the outlet blocks. When one is rebuilt, ask the roofer how it has been sized for the roof area and Sydney’s heavy-rain intensities, and where its overflow goes: a rainhead or an overflow sump, not the ceiling.

More on box gutters, fascia and gutter guard is on our gutters and downpipes page, and the gutter and downpipe calculator uses the building code’s tables for eaves gutters.

What to do when it starts dripping

Catch the water, keep away from wet light fittings and sagging plaster, and take photos with the date. In the roof space, the stain on the timbers next to a skylight shaft, or along the line of a box gutter, tells the roofer where to start. If a storm caused it, call the NSW SES on 132 500 and your insurer; see insurance and roof leaks.

Our advice is not to climb up and clear a box gutter in the rain. It is usually on a steep or high part of the roof, often with no safe edge, and SafeWork NSW treats work with a risk of a fall of more than 2 metres as high-risk construction work.

What skylight and box gutter repairs cost

We found no Sydney roofer that publishes separate prices for skylight flashings or box gutters. These leak-repair bands are the closest, each with its publisher:

Published prices for leak-type repairs

Quoted as each publisher states them, checked 1 October 2026; neither says whether GST is included. The first two rows are a Sydney-wide roofing contractor’s undated Mona Vale page, which uses the same wording as its pages for other suburbs. The last row is a Northern Beaches roofing contractor’s storm-damage page (published 30 April 2026), which calls its figures indicative. They’re per job, and the roofer prices yours once they’ve found the cause.

Resealing a flashing is one of the jobs in the simple-jobs band above; replacing a skylight’s whole flashing kit is more work. Rebuilding a rusted box gutter, with new sumps and overflows, is bigger work and is priced once the roofer has seen the length, the access and what’s under it.

Who fixes skylights and box gutters

In NSW, roof plumbing covers metal roof coverings, guttering, downpipes and roof flashings, which takes in box gutters. Roof tiling covers tiles, valleys, sarking and battens. Both licences include putting in skylights and ventilators. A contractor licence is needed for work valued at more than $5,000 in labour and materials, including GST; check it on the Service NSW licence register.

Replacing a skylight with a bigger one, or cutting a new one, isn’t covered by NSW’s exempt re-roofing rules, which say the work must “not comprise the making of, or an alteration to the size of, any opening in a wall or roof, such as a doorway, window or skylight”. Ask about approval before choosing a larger unit.

Common questions